Heating, ventilating and air conditioning (HVAC) systems are in wide use for providing clean, comfortable working environments in buildings such as offices, schools and the like. Such systems "condition" air by regulating air temperature and relative humidity and, usually, by removing very small airborne particulates therefrom. Such systems use air from the outdoor ambient, from within the building (i.e., re-circulated air) or a system may use a combination of fresh and re-circulated air.
To regulate air temperature or humidity or to filter out airborne particulates, it is necessary that the air be moved across or through, e.g., cooling coils, heating units, filters and water vapor injectors or evaporators, the latter for increasing relative humidity. Primary air movement is usually effected by what is known as an air handling unit.
A large system for, say, a multi-story office building may have a number of air handling units, each of which may be as large as 10 feet or so in length, width and height. Each air handling unit is equipped with a very large blower not unlike (except as to size) a common household fan. Such blower has a fan blade driven by an electric motor.
Air forced from an air handling unit by such blower is directed along one or a few air ducts, the cross-sectional area(s) of which are relatively large. Such ducts are usually made of sheet metal and each duct is branched into two or more smaller ducts. Each of such smaller ducts is terminated (in the acoustical-tile ceiling of an individual room or small group of rooms, for example) by a diffuser. As its name suggests, a diffuser directs the conditioned air in different directions so that the flow of air will not be sensed or will scarcely be sensed by persons occupying the room(s).
In more modern systems, a variable air volume (VAV) terminal unit may be interposed between the duct and the diffuser. In such a terminal unit, the volume of air urged through the diffuser over a given length of time is controlled. VAV terminal units permit "personalizing" the temperature of a particular room or group of rooms to the likes of the occupants. U.S. Pat. No. 5,180,102 (Gilbert et al.) includes a general description of a HVAC system and U.S. Pat. No. 4,418,719 (Downs, Jr. et al.) describes a type of VAV terminal unit.
In conventional practice, a VAV terminal unit has a circular inlet collar mating with and connected to a run of duct from an air handling unit. A rectangular outlet end couples such terminal unit to a diffuser.
While there may be several sources of objectionable sound (i.e., "noise") in a HVAC system, at least every component of rotating machinery, e.g., the blower of an air handling unit, generates sound waves which propagate along the duct through the air flowing in the duct. And certain types of VAV terminal units include integral motor-driven fans. Unless attenuated to acceptable levels, the propagated sound waves are evident (and they may be very evident) to persons in the rooms served by the HVAC system.
Efforts to reduce or eliminate sound waves in air ducts are ongoing. Noise attenuators and silencers are described in U.S. Pat. Nos. 2,308,886 (Mason); 2,974,475 (Kristiansen); 3,033,307 (Sanders et al.); 3,511,336 (Rink et al.) and 4,287,962 (Ingard et al.).
(The apparatus of the Sanders et al. and Rink et al. patents are of a type known as "dissipative" devices since they rely at least in part upon sound absorptive material--"packing"--to attenuate sound. The apparatus of the Ingard et al. patent is of a type known as a "reactive" device which attenuates noise without using packing.)
The Rink et al. patent discloses a sound attenuator, the converging section of which is formed by two opposed curved panels. The resulting opening is rectangular when viewed in a plane normal to the long axis of the attenuator. And such attenuator has what the patent calls absorption chambers packed with an acoustical fill material. The flat diverging surfaces have holes that are 0.125 inches diameter and such holes are said to constitute less than 14% of the surface area.
The Ingard et al. patent discloses a packless silencer having a pair of opposed curved entry panels which are said to have a "semi-elliptical" shape. The resulting air entry port is rectangular and slot-like when viewed in a plane normal to the long axis of the silencer. The holes in the parallel flat panels are of uniform diameter for a particular thickness of sheet metal. Such diameters range from 0.032 to 0.188 inches diameter and a preferred percentage of the aggregate area of the holes to the total surface area is said to be in the range of 2.5 to 10%.
(Strictly speaking, the term "silencer" may somewhat overstate the capability of a device for reducing sound in a HVAC system. However, such term is widely used in the HVAC industry and is generally understood to mean a device which reduces sound to an acceptable level, perhaps even to a level imperceptible to most persons.)
At least in HVAC systems, sound attenuation is often determined by measuring sound power reduction in any one, some or all of eight octave bands which are described in more detail below. Such octave bands are used in the industry because they represent frequencies to which human hearing is most sensitive.
When a noise attentuator or silencer is used with a VAV terminal unit, conventional practice is to interpose a straight length of what might be termed "flow-normalizing" duct between the attenuator and the terminal unit. Such length of duct permits air flow (which is or may be disturbed by mere presence of the terminal unit or by its damper-like throttling valve) to re-establish a uniform velocity profile.
Traditional practice dictates that for air flow rates of 2500 feet per minute or less, the interposed straight length of duct should have a length at least 2.5 times the diameter of the duct feeding the VAV terminal unit in order to re-establish a uniform velocity profile. And it has been found that spacing the VAV terminal unit and an attentuator by a duct of such length reduces the sound level.
While prior art approaches to configuring and using noise attenuators have been generally satisfactory, they are not without some problems. For example, sound-deadening packing as described in the Rink et al. and Sanders et al. patents is troublesome to handle in the factory and install during manufacture. And in view of the invention, its presence makes the attentuator heavier and more unwieldly to mount than is otherwise necessary.
Another disadvantage of prior art practice involves the length of flow-normalizing intermediate duct placed between a VAV terminal unit and an attenuator. With such duct, the aggregate length of the VAV terminal unit, intermediate duct and attenuator is rather great.
An improved attentuator which addresses and overcomes some of the problems of prior art practice would be an important advance in the art.